Lithium-ion secondary batteries using a non-aqueous electrolytic solution have recently been adopted as a power source of portable electronic appliances because of their high voltage, high capacity, high output, and light weight. The lithium-ion secondary battery generally has a laminate of electrode plates prepared by winding a positive plate and a negative plate in a spiral form across a porous resin membrane having fine pores and functioning as a separator. The laminate of electrode plates is arranged in a cylindrical attery casing of stainless steel working as a negative electrode. The positive plate is an aluminum foil collector plate, on which an active material of positive electrode containing a lithium-containing composite oxide (for example, LiCoO.sub.2) is applied. The negative plate is a copper foil collector plate, on which an active material of negative electrode containing carbon is applied.
Commercially available lithium-ion secondary batteries include the laminate of electrode plates, which may have the structure obtained by laying one positive plate with active material films applied on both faces of an aluminum foil, one negative plate with active material films on both faces of a copper foil, and two separators one upon another in the sequence of the negative plate, one separator, the positive plate, and the other separator and winding the layers in a spiral form to arrange the negative plate as the outer layer. The laminate of electrode plates may alternatively have the structure obtained by laying one negative plate with active material films on both faces of a copper foil, one separator, two positive plates with an active material film on each one face of aluminum foils (wherein two aluminum foils are laid one upon the other to make the active material films exposed to the outside), nd the other separator one upon another in this sequence and winding the layers in a spiral form to arrange the negative plate as the outer layer.
Such lithium-ion secondary batteries conventionally have a safety valve, a temperature fuse, a PTC element, and the like, in order to ensure the safety in case that an abnormality in the circuit or wrong use causes a short circuit of the positive electrode and the negative electrode in the battery and thereby increases the internal temperature of the battery. A further safety measure is required to provide for a variety of working environments and unexpected accidents.
An abrupt internal temperature increase is occasionally observed, for example, in case that a sharp conductive member, such as a nail, is pierced into the battery casing in an over charging state, in case that an abnormal heat is externally applied to the battery, or in case that the battery is crushed in the heaping direction of the laminate of electrode plates.
In these cases, a short circuit apparently occurs between the positive electrode and the negative electrode in the battery. The reason of the abrupt internal temperature increase has so far, however, not been elucidated. The present inventors have found the reason of this phenomenon and completed the present invention.
In case that a nail or another sharp conductive member is pierced into the battery, the point of the nail becomes a negative electrode when passing through the battery casing functioning as the negative electrode, and comes into contact with the internal positive plate. This causes a short circuit across the nail. In case that an abnormal heat is externally applied to the battery, the separator composed of an organic material is fused first and thereby causes the positive plate and the negative plate insulated from each other by the separator to come into contact with each other and cause a short circuit. In case that the battery is crushed in the heaping direction of the laminate of electrode plates, a large stress is applied to the inner circumference of the laminate of electrode plates and breaks the separator, thereby causing the positive plate and the negative plate to come into contact with each other and cause a short circuit.
The lithium-containing composite oxide (active material of positive electrode) has a relatively high resistance among the constituents of the laminate of electrode plates under the condition of a short circuit. A flow of short-circuiting current accordingly increases the temperature of the lithium-containing composite oxide. The heat caused by the increased temperature accelerates a decomposition reaction of the organic solvent included in the battery. When a short circuit occurs in the battery in the charging state, the lithium-containing composite oxide under the charging condition falls into an unstable state with some release of lithium ions and is thus decomposed by the temperature increase to produce active oxygen. The active oxygen accelerates the reactions on the aluminum foil with the lithium-containing composite oxide applied thereon and in the organic solvent.
An object of the present invention is thus to provide a non-aqueous battery, which effectively prevents a short circuit between an active material of positive electrode and a negative electrode in case that an abnormal heat is externally applied to the battery, in case that a sharp conductive member, such as a nail, is pierced into a battery casing, and in case that the battery is crushed in a heaping direction of a laminate of electrode plates, and even when a short circuit occurs, prevents an increase in temperature of the active material of positive electrode due to the short circuit and thereby ensures the safety.